U. TORONTO (CAN) — The mere presence of a predator can cause enough stress to kill a dragonfly, even when the predator can’t actually get at its prey to eat it.

“How prey respond to the fear of being eaten is an important topic in ecology, and we’ve learned a great deal about how these responses affect predator and prey interactions,” says Locke Rowe, chair of the department of ecology and evolutionary biology at the University of Toronto.

“As we learn more about how animals respond to stressful conditions—whether it’s the presence of predators or stresses from other natural or human-caused disruptions—we increasingly find that stress brings a greater risk of death, presumably from things such as infections that normally wouldn’t kill them,” says Rowe.

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For the study published in the journal Ecology and highlighted in Nature, Shannon McCauley, a post-doctoral fellow, and professors Marie-Josée Fortin and Rowe raised juvenile dragonfly larvae (Leucorrhinia intacta) in aquariums or tanks along with their predators. The two groups were separated so that while the dragonflies could see and smell their predators, the predators could not actually eat them.

“What we found was unexpected—more of the dragonflies died when predators shared their habitat,” says Rowe. Larvae exposed to predatory fish or aquatic insects had survival rates 2.5 to 4.3 times less than those not exposed.

In a second experiment, 11 percent of larvae exposed to fish died as they attempted to metamorphose into their adult stage, compared to only two percent of those growing in a fish-free environment.

“We allowed the juvenile dragonflies to go through metamorphosis to become adult dragonflies, and found those that had grown up around predators were more likely to fail to complete metamorphosis successfully, more often dying in the process,” says Rowe.

The findings could apply to all organisms facing any amount of stress—the experiment could be used as a model for future studies on the lethal effects of stress.

The research was supported in part by grants from the Canada Research Chairs program and the Natural Sciences and Engineering Research Council of Canada.